Studies of the effects of neonatal infection with mouse thymic virus on the immune system showed that the virus has no effect upon B cells and has strikingly differential effects upon subpopulations of T cells; for example, cells producing direct GVH activity in thymus were depleted following infection with thymic virus whereas cells in thymus producing synergy when combined with lymph node cells were normal. A rabbit antiserum specific for antigens of xenotropic murine leukemia virus has been prepared and is being used in the presence of virus-related cell surface antigens in the lymphoid compartments of different strains of mice. In addition, the genetics of factors regulating the expression of intact virus is being evaluated. Other studies investigated the immune mechanisms responsible for production of neurologic disease in mice infected with lymphocytic choriomeningitis virus (LCM).